1. Field of the Invention
The present invention relates to a pipe-inspecting apparatus having a self-propelled unit which can move within a pipe to inspect the interior of the pipe.
2. Description of the Related Art
Japanese Patent Publication No. 51-15678 discloses an endoscope comprising an insertion section whose middle portion is a self-propelled unit. The self-propelled unit comprises bellows made of elastic material, and two balloons coupled to the ends of the bellows. The self-propelled unit can move back and forth within a tubular member in the following way. First, the rear balloon is inflated until it pushes the inner periphery of the tubular member. As a result, the bellows is held in the tubular member. Then, pressurized air is supplied into the bellows, thus expanding the bellows forward in its axial direction. Thereafter, the front balloon is inflated until it pushes the inner periphery of the tubular member. Next, the rear balloon is deflated, and the bellows is also deflated. As a result, the self-propelled unit moves forward, pulling forward the rear end portion of the insertion section. As the balloons and the bellows are repeatedly inflated and deflated in this manner, the self-propelled unit moves forward within the tubular member, and the insertion section advances forward in the tubular member.
The self-propelled unit disclosed in Japanese Patent Publication No. 51-15678 cannot move the insertion section in a tubular member when the insertion section is relatively long and heavy. More precisely, the force of the expanding bellows is not strong enough to push the insertion section, and the force of the contracting bellows is insufficient to pull the insertion section.